1. Field of the Invention
The invention relates to apparatus for the thermoplastic shaping of thermoplastic polymeric resins and more particularly relates to such apparatus including an improved means of maintaining the thermoplasticity of the resin during shaping and minimizing the cooling period.
2. Brief Description of the Prior Art
The prior art literature is replete with descriptions of apparatus for thermoplastically processing thermoplastic, synthetic, polymeric resins; see for example the descriptions given in the U.S. Pat. Nos. 2,859,476; 3,308,508; 3,404,432; 3,820,927; 3,402,427 and 4,512,730.
In general, such apparatus includes resin extruders, inflection molding apparatus, blow-molding apparatus, thermoplastic resin coating devices, resin laminators and like apparatus. Processing in such apparatus may include mixing, plasticizing, shaping or simply carrying the resin in a thermoplastic state from one point to another point.
In common to all of the above-described apparatus and devices for processing thermoplastic polymeric resins are associated means for obtaining and, maintaining thermoplasticity of the resin during processing, such as the shaping of the resin material into an article. Maintenance of thermoplasticity may require both heating and cooling of the resin at various times in the processing of the resin. Cooling may be needed to avoid thermal degradation of the resin. Almost all of the resin processing techniques rely at least in part on heating or cooling of the polymeric resin by thermal transfer through the agency of a metal surface, part of the processing apparatus. Heat is generally applied to an outside surface of the metal apparatus by concentrated heat sources such as band heaters, or from within the body of the metal part by heater rods or circulating, heated fluids such as air, water or other chemical liquids. In all cases, the metal heat transferring components have to be of substantial thickness and mass to resist extreme pressures and mechanical forces. The large metal mass responds slowly to changes in thermal input or cooling so that precise control of narrow temperature ranges within a mold is difficult to control as well as the achievement of rapid cooling and heating cycles. Also, when temperature differences are desired in adjacent zones of the same mold, it is difficult to localize the particular and different temperatures to be maintained for appreciable periods of time. This shortcoming is especially troublesome for relatively complex molding techniques and apparatus, such as in the molding of large parts.
The improved mold apparatus of the invention includes as a means for controlling thermoplasticity of a resin within a mold, thin film heaters which may be mounted in close association with the thermoplastic polymeric resins being processed in the apparatus. Heavy metal components to achieve thermal transfer to the resin are not necessary. There can be a saving of weight, materials and labor in manufacture. With the closer juxtaposition of the heating element to the shaping plastic, a closer control of resin temperature is maintainable with quicker response times to maintain a pre-determined resin temperature, even in adjacent but different zones or localities of the same mold. The lower thermal mass of the mold elements is more responsive to cooling or changes from heating to cooling or cooling to heating.